A living radical polymerization wherein growing termini of the polymer have an activity that enable their chemical conversion [for example, atom transfer radical polymerization (ATRP) (see Non-patent Document 1); nitroxide-mediated polymerization (NMP) (See Non-patent Document 2); or reversible addition-fragmentation chain transfer polymerization (RAFT) wherein a sulfur compound is mediated (See Non-patent Document 3)] has attracted a great deal of attention over the last decade because, in such a method, the molecular weight of the polymer, order of monomer residues, the multidimensional structure, etc. could be freely controlled as it was different from a conventional radical polymerization In particular, it has been demonstrated that an atom transfer radical polymerization system wherein a metal complex is combined with a halogen compound is applicable to a wide range of monomer types, and a method of precisely controlling a monomer using such a system has also been applied to chemical modifications of a surface or boundary of a substrate, or device construction other than polymer synthesis.
With regard to metal catalysts used in the ATRP method, their central metal is generally copper or ruthenium. Such catalysts do not have a clear structure of a metal complex, and a metal ion and a compound that functions as its ligand (for example, amines) are often mixed into a polymerization reaction system to form such catalysts. In such a polymerization system, the catalytic activity of the metal is exhibited after the metal bonds to the ligand in the system, thereby forming a complex. When the ligands and the metal ions are used by mixing them into the polymerization reaction system, not all metals can form complexes. The metals that do not form a complex cannot show catalytic activity. Accordingly, the catalytic efficiency of the metal will be lowered. This brings about disadvantages wherein it may be required to increase the metal concentration or such a low activity may be insufficient for production of a high-molecular-weight polymer. Furthermore, the increase of the metal concentration will present a further a burden in requiring a step of removing the metal after the polymerization reaction, and may also raise a problem of environmental pollution due to the toxicity of metals. Additionally, it is also required to use an excessive amount of amine ligands, among others (see Patent Document 1 and 2). Many problems such as difficulty in controlling the reaction if the type of monomer is changed in the polymerization reaction; or inclusion of compounds other than monomers making polymer purification complex, have been mentioned when an excessive amount of amine ligands is used.
In general, an active halogen organic compound is used as a polymerization initiator in the ATRP method. In addition, when a polymerization is conducted where the active halogen organic compound is substituted with a conventional radical generator (for example, a peroxide-radical generator, or an azo-based radical generator) in the ATRP method, such a method is called “reverse-type ATRP” (R-ATRP). According to the R-ATRP method, a metal catalyst is added in a conventional radical polymerization process whereby reactive residues can be incorporated in termini of the polymerized products, and synthesis of a block copolymer can be achieved using such residues. Thus, the R-ATRP method can produce a polymer whose structure is controlled in the existing production process without using a halogen initiator, and therefore, this method is useful. In the R-ATRP method, many techniques wherein a copper ion complex formed basically using an amine as a ligand is used are known. Consequently, the R-ATRP also has the same problems as in the ATRP method such as the requirement of increasing the metal ion concentration or the ligand concentration, a decrease in the catalytic efficiency, complexity of the polymer purification, or polymer coloration.
In a living radical polymerization using a metal complex, the production of a polymer using a safe and cost-effective iron catalyst has attracted much attention in terms of environmental protection (Non-patent Document 4).
In the ATRP method, a polymer production technique wherein an iron ion and a ligand (for example, amines, phosphine compounds, or phosphite esters) are mixed with a polymerizable monomer to conduct a polymerization; or a polymer production technique wherein a synthesized iron complex and a polymerizable monomer are mixed to produce a polymer has been disclosed (see Non-patent Document 5: Sawamoto et al, proceedings for the 53th polymer symposium 2004, 2B16 on page 2456). For example, a method of polymerizing a methyl methacrylate wherein a bivalent iron ion and an amine-based ligand are mixed with a monomer, and a halogen initiator is used (see Non-patent Document 6); or a method of polymerizing methyl methacrylate wherein an iron complex formed using a bivalent iron ion and a phosphorus compound as a ligand, and a halogen initiator are used (for example, see Non-patent Document 7 or Patent Document 3) has been reported. However, in ATRP methods using such iron catalytic systems, a high-molecular-weight polymer, whose molecular weight reaches one hundred thousand units, is rarely synthesized. Additionally, in such methods, the iron complexes cannot be recovered and recycled for the polymerization reaction.
In the R-ATRP, use of an environmentally-friendly iron ion compound as a catalyst has been studied. For example, polymerization of methyl methacrylate using a complex of FeCl3 and isophthalic acid as a catalyst has been reported (see Non-patent Document 8). In this system, a polymer having a molecular weight of up to about fifty thousand could be obtained. However, it is required to use N,N-dimethylformamide, which is strongly suspected of having carcinogenicity, as a reaction solvent. Moreover, polymerization of methacrylate or styrene wherein an iron complex formed with an organic onium cation and an anionic iron chloride-based compound is used as a catalyst has been reported (see Non-Patent Document 9). However, the R-ATRP radical polymerization using such an iron complex or iron compound has many problems to be solved (for example, the molecular weight of the polymer stays at twenty or thirty thousand units, or it is difficult to control a block copolymer).
On the other hand, in the living radical polymerization system using a metal catalyst, removal of metals from the polymer after polymerization has been considered as a key issue. In a sense, removal of the remaining metals from the polymer is a real issue to achieve practical use of the living radical polymerization rather than the polymerization reaction thereof. To remove the metals, for example, a method utilizing a complexing agent in a polymer-purification step has been studied (see Patent Document 4 or 5). The use of environmentally-friendly iron ion compounds as catalysts is highly advantageous in the entire process of the polymer production including a step such as an additional treatment because they are nontoxic, compared to the other metal ions such as copper, cobalt and ruthenium. However, the living radical polymerization using an iron ion has a disadvantage such as low polymerization efficiency, compared to a copper ion complex system. Furthermore, disadvantages in the production process such as instability of the ion catalyst or difficulty in recycling the iron catalyst have been mentioned.
With regard to the living radical polymerization reaction, the development of a polymer production method wherein iron complexes having a higher catalytic activity are used to conduct the polymerization reaction, and the iron complexes are removed from the polymerization reaction system, and recovered by an easy method without disposing of them, thereby recycling them as polymerization reaction catalysts is considered to be a very important issue from a viewpoint of industry.
Patent Document 1: Japanese Unexamined Patent Publication No. H8-41117
Patent Document 2: Japanese Unexamined Patent Publication No. 2002-80523
Patent Document 3: Japanese Patent No. 2,946,497
Patent Document 4: Japanese Unexamined Patent Publication No. 2002-356510
Patent Document 5: Japanese Unexamined Patent Publication No. 2005-105265
Non-patent Document 1: J. Wang et al. 1995. Macromolecules 28, page 7901
Non-patent Document 2: C. J. Hawker et al. 1996. Macromolecules 29, page 5245
Non-patent Document 3: J. Chiefari et al. 1998. Macromolecules 31, page 5559
Non-patent Document 4: Matyjaszewski et al. 2001. Chem. Rev. 101, page 2921
Non-patent Document 5: Sawamoto et al., Proceedings for the 53th polymer symposium 2004, 1Pb022 on page 136
Non-patent Document 6: Matyjaszewski et al. 1997. Macromolecules 30, page 8161
Non-patent Document 7: Ando et al. 1997. Macromolecules 30, page 4507
Non-patent Document 8: Zhu et al. 2001. J. Polym. Sci. Part A: Polym. Chem. 39, page 765
Non-patent Document 9: M. Teodorescu et al. 2000. Macromolecules 33, page 2335